1. Technical Field
The present invention relates to an elemental layer on organic film product and the method and apparatus for applying the elemental layer. More specifically, the invention disclosed herein pertains to an inorganic layer that serves to keep polymer film from welding to itself when rolled or stacked but can also serve as an interface for future functionalization. This nanolayer can be formed during the original manufacturing of the polymer film by the use of chemical vapor deposition apparatus and is compatible with methods for depositing anti-block, primer, and/or high quality barrier layers on the surface of a film substrate to improve characteristics of the film substrate.
2. Description of Related Art
Multi-layered film structures made from petroleum-based products, polymers, copolymers, bio-polymers and paper substrates are often used in flexible films and packaging structures where there is a need for advantageous barrier, sealant, and graphics-capability properties. Barrier properties in one or more layers comprising the film are important in order to protect the product inside the package from light, oxygen and/or moisture. Such a need exists, for example, for the protection of foodstuffs that may run the risk of flavor loss, staling, or spoilage if sufficient barrier properties are not present to prevent transmission of light, oxygen, or moisture into the package. A graphics capability may also be required so as to enable a consumer to quickly identify the product that he or she is seeking to purchase, which also allows food product manufacturers a way to label information such as the nutritional content of the packaged food, and present pricing information, such as bar codes, to be placed on the product.
In the packaged food industry, protecting food from the effects of moisture and oxygen is important for many reasons, such as, health safety and consumer acceptability (i.e., preserving product freshness and taste). Conventional methods to protect food contents incorporate specialized coatings or layers within or on a surface of the substrate which function as an impervious barrier to prevent the migration of light, water, water vapor, fluids and foreign matter into the package. These coatings may consist of coextruded polymers (e.g., ethyl vinyl alcohol, polyvinyl alcohol, and polyvinyl acetate) and/or a thin layer of metal or metal oxide, depending on the level of barrier performance required to preserve the quality of the product stored in the package substrate.
Coatings produced by chemical vapor deposition are known to provide certain barrier characteristics to the coated substrate. For example, an organic coating such as amorphous carbon can inhibit the transmission of elements such as water, oxygen and carbon dioxide. Accordingly, carbon coatings have been applied to substrates, such as polymeric films, to improve the barrier characteristics exhibited by the substrate. Another example of coatings applied to substrates to improve barrier adhesion performance includes coatings comprised of inorganic materials such as inorganic metal oxides. Ethyl Vinyl Alcohol (EVOH) and other polymer skin layers are widely used to prime or improve the wettability of film substrates for the application of a barrier layer (also referred to herein as “metallization primer”). Aluminum metal, aluminum oxide, and silicon oxide are widely used for the direct application of barrier layer(s) directly to the substrates (also referred to herein as “metallization”). Aluminum oxides and silicon oxides also provide abrasion resistance due to their glass-like nature.
The inorganic coatings described above may be deposited on to substrates through various techniques as known in the art. Such techniques include vapor deposition, either physical vapor deposition (PVD) or chemical vapor deposition (CVD). Examples of PVD include ion beam sputtering and thermal evaporation. Examples of CVD include glow discharge, combustion chemical vapor deposition (CCVD) and plasma enhanced chemical vapor deposition (PECVD). All such coatings are now made in a secondary process after the film has been formed and either wound or stacked.
The most commonly known and utilized method for depositing barrier layers on film packaging substrates for metallization requires the use of a vacuum chamber to provide the vacuum environment for the deposition of inorganic atoms/ions on to the film substrate surface. This known technique, as used in the food packaging industry, consists of processing packaging film rolls which are from less than 1 to three meters wide and 500 to 150,000 meters in length running at industry speeds of 60-300 meters/min in a vacuum metallization chamber. This equipment is highly specialized, requires a great deal of electrical power and is capital intensive. Current vacuum chamber processes for metalizing films is inefficient in many respects due to the high capital/operating costs and limited operational/production capacity associated with the use of such equipment, and the requirement to use high end film to achieve the desired barrier.
Combustion chemical vapor deposition (CCVD) and plasma enhanced chemical vapor deposition (PECVD) apparatus and methods are known in the art, as disclosed in U.S. Pat. Nos. 5,997,996 and 7,351,449, the disclosures of which are hereby incorporated by reference. Typically, a combustion flame or plasma field provides the environment required for the deposition of the desired coating via the vapors and gases generated by the combustion or plasma on to the substrate. The elemental precursors (e.g. organometallics) may be vaporous or dissolved in a solvent that may also act as a combustible fuel. The deposition of organic and inorganic oxides may then be carried out under standard and/or open atmospheric pressures and temperatures without the need of a vacuum chamber, furnace and/or pressure chamber.
As described above, the application of barrier to food packaging is required to protect food and food products from the effects of moisture and oxygen. It is well known in the art that metalizing a petroleum-based polyolefin such as OPP or PET reduces the moisture vapor and oxygen transmission through specialty film by approximately three orders of magnitude. Conventional technology is to employ an inorganic layer of metal or ceramic on a special polymer film. The inorganic layer may be aluminum, silicon, zinc, or other desired element in a metal or oxide form. However, the surface of the substrate on to which the barrier layer will be applied typically needs to be primed to increase its surface energy so as to be receptive to the deposition of the metal barrier to be deposited thereon and/or to “smooth” the surface to be metalized so as to reduce the surface gauge variation or surface roughness of the film to be metalized. The term “wettability” is defined herein to include surface energy, metal adhesion bond strength, and any other associated characteristic which would increase the receptiveness of the film layer surface for deposition of coatings.
For example, the utilization of aluminum metal as a barrier layer on low energy plastics, such as biaxially oriented polypropylene (BOPP) film, requires a metallization primer to reduce the gauge variation of the film substrate surface and/or to improve the adhesion or bond between the metal and film substrate. Various chemical methods are employed to prime the substrate surface layer for improving the substrate surface and/or bonding of the metal barrier layer to the film substrate. With polymer film substrates, one method to prime the substrate for metallization is to co-extrude a specialized polymer as a skin layer on the substrate film. These skin layers may comprise ethyl vinyl alcohol (EVOH), polyvinyl alcohol (PVOH), and polyvinyl acetate (PVA), Ethyl Vinyl Acetate (EVA), Polyethylene Terephthalate Glycol (PETG), amorphous Polyethylene Terephthalate (aPET), among other polymers used in the industry. Unfortunately, these materials are quite expensive and add additional cost to the manufacture of metallization ready films. Also having multiple polymer compositions reduces recyclability of the product.
Plastic film cores, such as OPP, polystyrene (PS), and polyethylene terephthalate (PET) are typically treated with corona discharge or flame treatment. This helps increase wettability. However, these treatments tend to create undesired, adverse impacts on film substrate characteristics such as the formation of pin holes, chemical degradation of the surface through cross linking or intramolecular chain scission that can adversely affect downstream metallization and heat sealing processes.
After formation, the film substrates are typically wound around a core into a roll for storage and distribution. Additional additives, such as slipping agents, anti-statics and anti-blocks as previously described, are usually incorporated into substrate films before winding and migrate to the surface of the film substrate in order to prevent or minimize blocking, welding or “sticking” of the film surfaces when the film is wound. The addition of conventional slip and/or anti-block additives interferes with establishing an effective metalized barrier layer and tends to degrade the performance of the film substrate, as the anti-block additive particles, along with other environmental particles such as dust, are transferred from the sealant layer of the film to the metallization surface layer during the winding process. The presence of these particles increases the surface roughness, surface gauge variance of the film, and forms holes or gaps in the metalized layer later deposited. Slip agents and anti-statics decrease the wettability of the film surface for metallization and further degrade the metal adhesion and barrier potential of the film.
As such, there exists a need for a polymer film product that does not contain such additives, but does not weld to itself and can still be processed on conventional film web handling equipment. To accomplish this end, there is a need in the art for a more efficient and economical apparatus and method to prime a substrate for metallization. Likewise, a need exists in the art for an improved apparatus and method for improving the barrier of a substrate which is less expensive and more energy efficient than tradition metallization, while achieving and maintaining high quality barrier characteristics. Additionally, a need exists in the art for an improved apparatus and method for treating film substrates without the need for the addition of conventional anti-block or slip agents to reduce blocking of the film in an in-line manufacturing environment.